1. Field of the Invention
This invention relates generally to a fuel cell and, more particularly, to a fuel cell that includes a thin-sheet diode electrically coupled to bipolar plates in the cell for preventing a polarity reversal of the fuel cell.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid-polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of each MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of each MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells from one cell to the next cell as well as out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The polarity of an individual fuel cell within the fuel cell stack can be reversed (cell overload) if the stack load attempts to draw more electrical current from the stack than the cell can generate. Because the cells are electrically coupled in series, a low performing cell may experience cell overload if the normal operating cells are able to supply a high stack load. In this condition, the cathode side of the bipolar plate becomes more negative than the anode side of the bipolar plate in the cell causing reverse polarity. Therefore, the output current available from the stack is defined by the weakest performing cell in the stack.
Cell overload can occur if the anode side of the fuel cell is starved for hydrogen, i.e., the amount of hydrogen being provided to the cell is not enough to provide the desired power output. In this situation, the fuel cell may begin oxidizing the bipolar plate and the carbon instead of the hydrogen in the hydrogen flow channel. This condition may be permanently detrimental to the performance of the overloaded cell, and in turn the entire stack. It is also possible to reverse the polarity of the fuel cell by starving the cathode side of the cell of air. Reversing the polarity of the cell in this manner causes hydrogen evolution to occur on the cathode side of the overloaded cell. The simultaneous presence of hydrogen and oxygen on the cathode side is undesirable and can result in cell overheating and damage due to direct reaction.
The above described reverse polarity condition is prevented in known fuel cells by monitoring the voltage of each fuel cell in the stack, and increasing the hydrogen or air flow or reducing the load if the potential of any one of the cells drops to zero potential. For example, more hydrogen could be provided to the hydrogen flow field in the stack to increase the performance of the low performing cell. However, because the anode gas flow is in parallel with each cell, the extra hydrogen is wasted for those cells that are operating properly.
It is known in the electrical art that a diode can be electrically coupled in parallel with any other electrical device to limit the voltage applied to that device when power is applied from a source. A diode can effectively protect the device by allowing electrical current to flow around the device at some threshold voltage that is characteristic of the particular diode selected, instead of allowing the full power of the connected source to be applied to the device.
It is possible to electrically couple a diode in parallel with each fuel cell in a fuel cell stack to protect the stack by preventing the potential of the cells from dropping more than one forward diode voltage below zero. Selecting a diode that has a low forward voltage (as low as 0.1-0.2 V) means that at cell voltages greater than zero the diode looks like an open circuit, and electrically appears as if it isn't in the circuit. If a fuel cell voltage is dragged below zero past the forward voltage of the diode, the diode will become conductive. This would prevent the cell from going any lower than the negative forward voltage drop, and would cause electrical current to be routed from one bipolar plate to the next bipolar plate around the overloaded MEA in the cell. The diode will remain conductive until the load is lessened or removed from the stack, at which point the voltage of the cell would rebound, the diode would turn off, and the cell would be able to continue to operate at lighter loads.
There are a number of difficulties in implementing such a diode in a fuel cell stack, including handling the electrical currents in a typical stack. Currently, no diodes exist that are physically sized and shaped to fit into the cells of a fuel cell stack. Further, stack currents are typically too great to send the current outside of the stack so that the current could be routed through a diode external to the stack because the cabling required to do this would be too bulky.